pgsql: Replace alter_table.sql test usage of event triggers.

Replace alter_table.sql test usage of event triggers.

The test in 93765bd956b added an event trigger to ensure that the
tested table rewrites do not get optimized away (as happened in the
past). But doing so would require running the tests in isolation, as
otherwise the trigger might also fire in concurrent sessions, causing
test failures there.

Reported-By: Tom Lane
Discussion: https://postgr.es/m/3328.1570740683@sss.pgh.pa.us
Backpatch: 12, just as 93765bd956b

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ae5cae54ca6b1949829026b9fbb744c7f5a28bd5

Modified Files
--------------
src/test/regress/expected/alter_table.out | 105 +++++++++++++++++++++---------
src/test/regress/sql/alter_table.sql | 71 ++++++++++++--------
2 files changed, 116 insertions(+), 60 deletions(-)